A culture of silence surrounds menstruation while inadequate facilities predispose adolescents to psycho-social trauma and cyclic absenteeism from schools. This study assessed the knowledge and menstrual hygiene management (MHM) practices among in-school adolescents in an urban city in Nigeria. The descriptive, cross-sectional study identified 400 respondents through a multistage technique and collected data through validated questionnaire and observational checklist. The mean age and age-at-menarche of respondents were 15.3 ± 1.5 and 12.8 years, respectively. Most respondents (70%) were aged 10–15 years, had good knowledge of MHM (296, 74%) and knew about menstruation before menarche (85.4%). MHM knowledge was significantly associated with: mother's education (p = 0.029); absorbents changing frequency (p = 0.003); and age-at-menarche (p = 001). The number of absorbents used daily was 2.5 ± 0.7; 90% of adolescents changed absorbents at least twice daily while 24.2% had previously changed it in school. Moreover, 14.4% of respondents abstained from school during menstruation and there was a significant association between school type and menstrual absorbents used (p = 0.0001), mothers' education (p = 0.0001) and disposal of used absorbents (p = 0.004). Spent absorbents were mostly disposed of in pit latrines (35.1%) and by burning (32.6%). A wide disparity remains between good MHM knowledge and poor practices. Therefore, gender-friendly facilities should be provided in schools to ensure retention of girls and end psycho-social trauma experienced during menstruation.